The present invention relates generally to MR imaging and, more particularly, to a method and apparatus to graphically display a pre-scan volume on a localizer image.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
During fabrication and construction of the magnet assembly for an MR assembly, manufacturing tolerances and deviations in material make-up results in an inhomogeneous B0 field being created by the magnet assembly absent shimming. As a result of the magnet manufacturing process, it is not uncommon for the magnet to produce an inhomogeneous field ranging from several hundred parts per million (ppm) to several thousand ppm, and a non-accurate center magnetic field that is significantly out of range. The importance of these variations is glaringly apparent given that MR systems require an intense uniform magnetic field, typically less than 10 ppm of variations within a 40-50 cm spherical volume, but also an accurate center magnetic field value, typically less than 0.5% variation.
Typically, a pre-scan is carried to at least identify areas of field inhomogeneity so that shimming and/or other corrective measures may be taken to remove or reduce the inhomogeneities. Shimming is a field homogeneity corrective process that is important for MR systems because the average B0 field strength must be within a certain window for the RF hardware of the system. The shimming process includes the precise placement of shim elements within the magnetic assembly such that numerous small magnetic fields are generated to offset variations in the B0 field. The shim elements include active shims such as shim coils or permanent magnets as well as passive shims such as iron cores. Shim coils are also common in superconducting magnet assemblies and their shimming may be controlled by regulating current thereto. The shimming characteristics of permanent magnets may be controlled by regulating the mass and polarity of the magnet and the shimming effect of iron cores may be controlled by regulating the mass of the iron incorporated into the magnet assembly.
In MR applications, such as bilateral breast imaging, it is important that the center magnetic field and the volume-of-interest be substantially coextensive. For instance, for bilateral breast imaging with improved image quality, the center magnetic field should be positioned relative to both breasts. A number of tools have been developed to assist radiologists and MR technicians with the shimming process. For example, one such tool allows the radiologist to graphically position a 3D slab on a particular anatomy of a localizer image. After identifying and defining scan parameters to acquire MR data of the 3D slab, the radiologist is required to carry out a manual pre-scan to note down shim values and a center frequency high for data acquisition of the identified 3D slab. The radiologist then repositions the 3D slab on another anatomy from which data is to be acquired. Once again, the radiologist defines the scan parameters for data acquisition of the another anatomy. The radiologist then carries out a manual pre-scan to note down shim values and a center frequency high for the another 3D slab. From the shim values and center frequency high acquired for both anatomies, the radiologist must manually determine an average of the values. The radiologist then repositions the 3D slab to cover both of the designated anatomies. Scan parameters are then set for data acquisition of both anatomies in a single data acquisition followed by another manual pre-scan. In this manual pre-scan, the radiologist uses the average shim values and center frequency high previously calculated. Ideally, the average shim values and center frequency high are optimized for data acquisition of the designated anatomies in accordance with the user identified scan parameters. The radiologist then carries out the necessary shimming followed by data acquisition of the designated anatomies.
While the above tool as well as other tools have improved image quality, scan time has increased which negatively affects patient throughput, increases patient discomfort, and increases the propensity for patient induced motion artifacts. Specifically, in the example of bilateral breast imaging, the radiologist must carry out three pre-scan series to achieve good image quality. It would, therefore, be desirable to have a system and method capable of optimizing shimming for data acquisition of multiple anatomies without multiple pre-scan series.